<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
            <script>
                 function f(){
                      var  a=document.getElementById("d");
                      d.querySelector(".c1").innerHTML="hellow"
                     d.querySelector(".c1").style.color="red";
                     d.querySelector(".c1").style.fontSize="100px";
                 }
            </script>
</head>
<body>
     <div id="d">
          <p class="c1">class=c1的第一个p元素</p>
         <p class="c1">class=c1的第二个p元素</p>
     </div>
     <input type="button" onclick="f()" value="修改第一个class=c1的内容">

</body>
</html>